The present invention relates to an antenna for radiating and receiving circular polarized electromagnetic signals in particular signals with microwave or mm-wave frequencies.
Such antennas are of particular interest for high data rate applications, such as wireless communication systems in the microwave or mm-wave regime. Typical applications of that type are satellite-earth-communication, indoor wireless LANS or outdoor LOS private links. These applications require large bandwidths which can only be granted in very high frequency regions as e.g. from 15 GHz to 60 GHz. The circular polarization is necessary in order to omit the requirement for the user to observe the orientation of the antenna.
Antennas providing circular polarization are described in the prior art. Planar antennas in this field mainly make use of a microstrip technology, In EP 0 215 240 B1 for example, a planar-array antenna for circularly polarized microwaves is described. This antenna comprises a substrate being sandwiched between two metal layers. Openings are formed in both of the metal layers. In these openings excitation probes are provided on the substrate. An antenna of this design has the disadvantage that the structure thereof is rather complex and that the probes have to be aligned accurately with the openings in the metal layers, in order to comply with the required tolerances. This complex structure and alignment requires additional manufacturing steps and advanced technology.
Therefore, the object of the present invention is to provide an antenna which allows applications into the mm wave frequencies with good efficiency and is simple in structure.
This object is achieved by an antenna comprising a planar dielectric substrate, comprising a front and a back dielectric face, at least one subantenna means, comprising a first and second element for radiating and receiving circular polarized electromagnetic signals and at least one transmission line means for transmitting signals from and to said at least one subantenna means, whereby the antenna is characterized in that the first and second elements of the subantenna means are slots arranged orthogonal to each other in a V-shape on the front dielectric face of the substrate and that the transmission line means is arranged on the back dielectric face of the substrate.
The main advantages of the antenna according to the present invention are its simple structure and the decoupling of the feed network from the radiating elements, i.e. the slots. The simplicity of this planar antenna structure is given by the fact that the feed line and the subantenna means are both formed on one dielectric substrate on opposite sides thereof. For the inventive arrangement, hence, already a single layer substrate suffices. An additional alignment of a path on an upper layer is therefore not required. Such alignments are mandatory for aperture coupled patch path antennas. The tolerance is very small for high frequencies and therefore such an alignment is a tedious task. The possibility of omitting such an alignment during manufacturing of the antenna allows the use of cheaper technology and thereby decreases the overall costs. Simple planar technology, printed technology and/or simple and cheap photo lithographic processing of prints can be utilized. The simple structure and low costs are a strong necessity for a commercial success of an antenna and are met by the inventive structure. In addition the inventive antenna of the planar printed type is very easy to integrate with active devices on the same substrate.
With the feed line, which in particular for array configurations may be connected to an additional feed network, being arranged on the opposite side of the substrate from the subantenna means, it is ensured that the radiation of the antenna is only determined by the subantenna means, namely the radiating slots, which are well controllable.
The feed line which can be of microstrip structure is preferably arranged on the opposite side of the substrate under an angle of 45xc2x0 to each of the slots. With this position of the feed line the coupling section can be perpendicular to the direction of the feed line, in order to allow an even distribution of the power between the two slots. With the structure of the subantenna means comprising two slots arranged orthogonal to each other and being arranged in a V-shape the vertical slot can radiate the horizontal component and the horizontal slot can radiate the vertical component of the electromagnetic signal. A circular radiation of the antenna can thus be obtained by this simple structure.
Further advantageous features of the antenna according to the present invention are defined in the subclaims.
In a preferred embodiment the first or the second element of the subantenna means is greater in length than the other. The elements of the subantenna means are the slots arranged in a V-shape orthogonal to each other. The slots preferably have a rectangular shape with a bridge portion connecting them at the meeting point of the V-shape. Other forms can, however, also be realized in the antenna according to the invention, provided that the shape of the slots allows the desired excitation of electromagnetic signals and the lines extending through the middle of the slots in their longitudinal direction are perpendicular to each other. In one embodiment of the invention the width of each of the first and second element of the subantenna means increases from their feeding side to the opposite side thereof. The slots hence each have a tapered shape with the central lines of the two slots extending in their longitudinal direction being perpendicular.
The total slot length, being the sum of both slots of the subantenna means, is approximately one guided wavelength in the slot. If however one of the two slots is longer than the other, the field excited within the total slot has a 90xc2x0-phase difference between the components in the vertical and the horizontal slot or arms of the V-shape. This leads to a phase shift of 90xc2x0 between the vertical and the horizontal component which are radiated by the horizontal and the vertical arm, respectively. Due to this phase shift a circular polarized radiation at the correct frequency of operation can be obtained.
The transmission line can have various designs in order to match the antenna. The feed line preferably represents a microstrip line. In one embodiment the transmission line may comprise a first line for to the first element of the subantenna means and a second line for to the second element of the subantenna means, said first and second line being coplanar to each other. In a further embodiment the feed line includes a tapered portion. This structure of the feed line is in particular advantageous for instances where the real part of the impedance cannot be tuned to the characteristic impedance of the feed. In these cases, when the real part of the impedance is low, a low impedance microstrip line is used in the coupling region and is matched through the taper structure to the desired microstrip line. Naturally any other kind of known matching structure can be used.
The subantenna means and the transmission line are arranged on a dielectric substrate, which preferably has a dielectric constant of xcex5rxe2x89xa71. Suitable material for the dielectric substrate is for example Teflon-fiberglass with a dielectric constant of 2.17. The subantenna means are slots which are preferably formed in a metal coated area on one of the faces of the dielectric substrate. They can be obtained by metallizing one side of the substrate and etching the slots into the metallic layer by known etching techniques. The feed structure is obtained by applying metal to the opposite side of the substrate in the desired shape.
The antenna according to the present inventions can advantageously further comprise a reflector means. This reflector means which is normally represented by a reflector plate or plane can be spaced to and parallel with the back face of the dielectric substrate. Between said reflector means or plate and said back face of the substrate, low loss material should be located. Even though the inventive antenna can be operated without any reflector means, such means can be added in order to enlarge the broadside gain of the antenna and to cancel the backside radiation.
The inventive antenna is in particular suitable for being arranged as an antenna element in a phase antenna array comprising a plurality of antenna elements. The planar phase antenna array can be obtained by arranging several subantenna means each including two perpendicular slots on one substrate and feeding this arrangement by means of a feeding network, located on the opposite side of the substrate. In such an array configuration, the advantageous of the present invention specifically come to fruition. The arranging of the feed line on the opposite side of the substrate from the subantenna means provides a possibility of decoupling of the feed network from the radiating structure. With conventional antennas, in particular in array configuration, spurious unwanted radiation components are observed from the feed network. These components greatly decrease the axial ratio and are therefore undesirable. In the antenna according to the present invention in contrast the feeding network is completely decoupled from the subantenna means and thus the radiation is only determined by the well controllable subantenna means, namely the radiating slots. Reflections from mulitpath effects will be significantly attenuated.